A rotary vacuum pump is a vacuum pump comprising a pump housing having an inlet port and an outlet port and a plurality of pumping stages arranged between the inlet port and the outlet port and suitable for pumping a gas from the inlet port to the outlet port.
Each pumping stage substantially consists of a stator ring which is integral with the pump housing and of a rotor disc which is integral with a rotating shaft that is centrally arranged in the pump housing and that is driven in rotation at high speed by a motor.
Depending on the kind of pump, higher or lower vacuum degrees can be obtained.
Turbomolecular pumps are suitable for generating a vacuum degree of the order of 10−7 mbar (i.e. 10−5 Pa).
With reference to FIG. 1, a turbomolecular pump 100 comprises a pump housing 102 in which an inlet port 104 and an outlet port 106 are defined, a plurality of pumping stages being arranged inside the housing 102 between the inlet port 104 and the outlet port 106. More particularly the turbomolecular pump 100 typically comprises a first region A at lower pressure, arranged closer to the inlet port 104 and comprising a plurality of turbomolecular pumping stages 108a, and a second region B at higher pressure, arranged downstream the first region A in the flow direction of the pumped gas and closer to the outlet port 106 and comprising a plurality of molecular drag pumping stages 108b. 
Both turbomolecular pumping stages 108a and molecular drag pumping stages 108b comprise respective stator rings 112a, 112b integral with the pump housing 102 and rotor discs 114a, 114b integral with a central rotating shaft 110, the stator rings and rotor discs cooperating with each other for obtaining a pumping effect.
FIG. 2 shows in detail a turbomolecular pumping stage 108a. As mentioned above, such pumping stage comprises a stationary stator ring 112a cooperating with a rotor disc 114a driven in rotation by the rotating shaft 110.
The function of the rotor disc 114a is mainly to pump the gas molecules, while the function of the stator ring 112a is mainly to successively change the speed distribution of the gas molecules, before they are intercepted and pumped by the rotor disc 114a′ of the following pumping stage.
Both the stator ring 112a and the rotor disc 114a are equipped with respective radial blades 116a, 118a, which are equally spaced in the circumferential direction and oriented with opposite leads with respect to the rotation plan in order to perform pumping of gas molecules.
The number of the rotor and stator blades and the geometry thereof strongly affect the performances of the pumping stage. More particularly, the axial clearance between cooperating stator rings and rotor discs has to be kept narrow in order to obtain satisfactory performances, namely in terms of attainable vacuum degree.
The rotor discs 114a together with their blades 118a are typically obtained by milling.
The stator rings 112a together with their blades 116a could also be obtained by milling. However, this technology is very expensive, so that stator rings are preferably obtained by stamping: the use of this technology imposes many limitations and constraints on the geometry of the stator rings, but it allows for a remarkable reduction of the manufacturing costs.
In many turbomolecular pumps the rotor discs of the turbomolecular pumping stages all have the same diameter.
However, there are specific applications where it is desirable to have rotor discs with different diameters.
FIG. 3 partially shows in a very schematic way a turbomolecular pump 200 of the prior art suitable for such applications. The region at lower pressure of this turbomolecular pump 200 comprises a first set I of turbomolecular pumping stages 208a′ with rotor discs 214a′ having a smaller diameter followed (in the flow direction of the pumped gas) by a second set II of turbomolecular pumping stages 208a″ with rotor discs 214a″ having a larger diameter.
As mentioned above, the rotor discs 214a′, 214a″ are carried by a common rotating shaft 210 and they are equipped with rotor blades 218a′, 218a″. 
Each turbomolecular pumping stage also includes a corresponding stator stage 212a′, 212a″ comprising a substantially cylindrical spacer ring 220a′, 220a″ which is integral with the pump housing 202 and which supports a corresponding bladed stator ring 216a′, 216a″. 
It is evident that in the arrangement shown in FIG. 3 the pump housing 202 and the stator stages 212a′, 212a″ integral therewith are configured so as to accommodate the diameter change of the rotor discs. More particularly, the stator stage of the pumping stage at the rotor diameter transition region comprises a spacer ring 220a* having an L-shaped cross-section in order to follow the outline of the pump housing wall.
However, such known solution is affected by a severe drawback since a wide axial clearance 222 is formed at the rotor diameter transition region, i.e. at the L-shaped spacer ring 220a. 
Such axial clearance involves a remarkable degradation of the pumping stage performances.
The axial size of the clearance 222 could be reduced by reducing the thickness of the spacer ring 220a*; however, the spacer ring 220a* has to be thick enough to ensure the mechanical stability thereof, which prevents any possibility to reduce the axial size of the axial clearance 222 beyond a certain limit.
In order to overcome the above drawback, it would be possible to provide at the rotor diameter transition region a stator stage having a specially designed geometry and comprising a spacer ring and a bladed stator ring made as single piece for reducing to a minimum the axial clearance.
However, for carrying out this kind of solution it would be necessary to manufacture the stator stage at the rotor diameter transition region by milling, which would increase the manufacturing costs.
Therefore, the main object of the present invention is to provide a rotary vacuum pump comprising a first set of pumping stages having rotor discs with a first diameter and a second set of pumping stages having rotor discs with a second different diameter wherein the axial clearance between the rotor discs and the cooperating stator stages of the pumping stages can be kept very narrow even at the rotor diameter transition region so as to optimize the pump performances, without entailing any increase in manufacturing costs. More particularly, the main object of the present invention is to overcome the drawbacks of prior art by providing a rotary vacuum pump comprising a pumping stage that is specifically designed for matching a change in the pump rotor diameter.
In other words, the main object of the present invention is to provide a rotary vacuum pump comprising a first set of pumping stages having rotor discs with a first diameter and a second set of pumping stages having rotor discs with a second different diameter and further comprising a pumping stage suitable for being arranged between the first set of pumping stages and the second set of pumping stages and designed so that the axial clearance between its rotor disc and its cooperating stator stage—as well as the axial clearance with the adjacent pumping stages—can be kept very narrow.
This and other objects are achieved by a rotary vacuum pump as claimed in the appended.